Car ECU, replace or repair

[Simon]

I was thinking more damage done by say related back e.m.f. over time or maybe extra strong due to some extra revving ?

[SeanB]

Not likely, the normal failure mode of the sensors is the sensor dies, not the management unit.

[jucole]

Here's a short troubleshooting guide:
 - turn ignition on
 - check the voltage between battery ground and pin 46 of ECU connector. it should be 0 V
 - check the voltage between battery ground and pin 7 of ECU connector at room temp (20?). it should be 2.9-3.1 V
 - check the voltage between battery ground and pin 7 of ECU connector at 80?. it should be 0.7-0.9 V
 - let as know of the result

I'm quite interested to see what this turns out to be. I'll put my money on either the temp sensor or a bad connection to the ecu box.

[Simon]

but the mystery is how can the diagnostics bus be providing a sensor reading if it's broke or the connection is faulty. The only other explanation is a second sensor and control circuit that is totally undocumented while the ecu is perfectly capable of running the fan as we found so that makes no sense...... I'll try and find the sensor when I get back over the weekend, but as the diagnostics bus gives a sensor voltage don't hold your breath on it being the sensor.

Does anyone know the cut in temp for the rad fan on a fiesta MK4 1.3L finesse ? I need to run the engine up to that for testing but want to keep as far from engine burn up temperature as possible.

[aluck]

Does anyone know the cut in temp for the rad fan on a fiesta MK4 1.3L finesse ? I need to run the engine up to that for testing but want to keep as far from engine burn up temperature as possible.

can't tell for fiesta, but it's 97-108 C for the majority of cars. of course, it's the coolant temp. engine is up to 3500C inside and on the surface - it depends on the exact place.

[Simon]

well I'm talking coolant temp, at 101C the diagnostics says it's overheating but it may be over reacting to save the engine ?

[0xdeadbeef]

Dunno about Ford, but usually the fan should kick in at 90-100°C. Let's say around 95°C. 100°C without a fan hints towards a defective thermostat if the fan itself is working via obd.
Are you sure the temperature read via obd is really the coolant temperature? This could be also the oil temperature or whatever.
If you locate the thermostat for the coolant temperature, you could try to remove it and see if the temperature changes. Or use coolant spray or a heat gun on it and see if the temperature changes.

[Baliszoft]

Or just simply measure resistance.

[benemorius]

but the mystery is how can the diagnostics bus be providing a sensor reading if it's broke or the connection is faulty. The only other explanation is a second sensor and control circuit that is totally undocumented while the ecu is perfectly capable of running the fan as we found so that makes no sense...... I'll try and find the sensor when I get back over the weekend, but as the diagnostics bus gives a sensor voltage don't hold your breath on it being the sensor.

There are most commonly two or more coolant temperature sensors - one for the instrument gauge, one for the ECU, one for an auxiliary cooling fan. The one for the cooling fan is usually a switch that operates one or more fan relays directly, while the others are full range thermistors. The fan can be turned on by either by the temperature switch directly or by the ECU. Very often the only reason that the ECU has an output to the fan relay is to turn the fan on when the aircon is on. Common sense and good practice might suggest that there need only be one sensor for everything, but I haven't ever seen less than two (yet).

Wiring diagrams in Haynes and similar manuals are more of a starting point than a complete reference. Even the rest of the manual will have omissions. If you want to know for sure what your car actually has, you have to get out there and look at it. I'd keep looking for another sensor. It ought to be in the radiator or somewhere along the output of it.

[SeanB]

A reason to think there is a thermoswitch while the ECU has a fan output is that the range does include models with an aircon, and the wiring loom is made for the top range then installed, with only the parts omitted for the base model. This is done to keep from having 2 near identical looms in stock on the line, and allows the decision on which spec is needed to be deferred to further down the line to the point where the dashboard in inserted, rather than at the front of the line. Cost of the extra loom is minimal compared to the extra pick bin, stock shelf ( these looms are supplied on a big board neatly folded so they are not stressed during install or storage) and chances for a mixup in production.

[aluck]

Dunno about Ford, but usually the fan should kick in at 90-100°C.

on a majority of Russian produced cars the fan kicks in at 103-107C. it's a NORM. and a coolant is kept under pressure to prevent it from boiling.

sometimes I think that those car's engineers have a slight form of dementia.

[Simon]

I'm guessing that fan cut in temperature is usually about the same temp above the thermostat opening temp, so with 88-90 degree thermostat opening where does that put fan cut in ?

[aluck]

I'm guessing that fan cut in temperature is usually about the same temp above the thermostat opening temp

Usually that's a different temperature. AFAIR, thermostat opens at 60-70C.

I was wrong. It's about the same temp.

[Simon]

yea my thermostat opens at about 88-92C and then the temp stabilizes around 85-89C, so much cut in by 98C ?

[aluck]

yea my thermostat opens at about 88-92C and then the temp stabilizes around 85-89C, so much cut in by 98C ?

if you are in a traffic jam, just opening thermostat is not enough, and temperature will go higher. then the fan will cut in.

[ot1]

@Simon,
I only read part of this thread, but I would say that the ECU is NOT the problem.
I would guess its a temp sensor.   get the OEM electrical schematics for your car AND THE CORRECT YEAR!!, do not use Chilton or Haynes, they are super crap, full of mistakes and very abbreviated, unreliable for electrical truobleshooting.

Many cars do have two water temp sensors check for this.

With your friend's OBD tool get into the car's realtime data, read the coolant temp, run the engine and watch the temp climb on the tool and pull the wire off the sensor, notice the change, if it doesn't the sensor is bad.  Two wires are probably in the sensor connector, one is ground, (brown?) jumper the connector, read the temp.

It is possible there is another control loop outside of the ECU control.  You can understand this by looking at the fan relay, for additional connections to the relay coil.

you can obtain OEM manuals from Ebay inexpensively, on DVD.  Don't buy just the schematics, because they may not be of your car, but with the whole manual to read, you will know.

Get on a forum for your car and see if they can help too.